The present invention relates in general to communication systems and, more particularly, to a method of identifying a sinusoidal burst frequency in a communication channel.
Cellular and paging communication systems have become widespread in much of the industrialized world. Cellular communication principally involves voice communications between a mobile station and a base station. The base station interfaces through a switched telephone system to complete the call. Paging systems transfer data, e.g. phone numbers and other text, from a base station to the pager. The distinction between cellular phones and pagers is rapidly diminishing. Cellular phones now, or soon will be able to, receive digital data that can represent voice, text, faxes, phone number, etc. Pagers offering voice messaging can receive text as well as voice data and respond in kind.
Most, if not all, cellular and paging communication systems operate on a set of standards that regulate the transmission frequencies, protocols, and other communication specifications. In Europe for example, one telecommunication standard is known as Group System Mobile (GSM). The GSM network standard is available in other countries as well.
Under GSM, the base station transmits modulated radio frequency (RF) information at particular points in time. When a user switches on a mobile cellular phone to communicate with a base station, the mobile phone must establish a time synchronization with the base station to properly receive the information. The mobile phone has an internal frequency reference, typically a crystal oscillator, that varies with temperature, time, and environmental conditions. The crystal oscillator provides a reference frequency to one or more phase lock loops (PLL) that generate local oscillator frequencies to downconvert the received RF signal. The PLL local oscillator frequency must be precisely matched to accurately receive the modulated RF information.